1. Field of the Invention
The present invention relates to electroless plating compositions, more particularly, to electroless plating compositions for applying metal films to substrates having smooth surfaces i.e. glass or ceramic surfaces, without using hazardous chemicals or extreme conditions. The present invention is also directed to a process for forming electroless plating compositions and a method of applying these compositions to a substrate.
2. Description of the Prior Art
Multi-layer laminates of metal and dielectric materials are useful in various applications. For example, such coatings may be used as thermal barriers applied to substrates subjected to high temperatures, and as conductive coatings applied to electrical resistance elements.
Traditionally, metallic coatings are applied to substrates in one of three different ways. In the first process, the so called wet method, a slurry of metallic filler and a cementitious or sinterable metal binder is sprayed, in a suitable aqueous or organic solvent, to a desired thickness on the substrate. The substrate is then fired to remove the solvent and any organic binders, and to form or sinter the ceramic binder. In the second process, the so-called dry spray method, the metallic ingredients are sprayed without solvent at a high velocity onto the substrate through a flame or plasma which causes the component materials to melt prior to impact with the substrate. In the third process, the surface of the substrate to be coated (e.g. glass, ceramic or polished metal) is pretreated and a sol-gel is applied to the pretreated substrate to produce solid films in the form of monoliths, thin films, particles or fibers.
These conventional methods suffer from several shortcomings. For example, the traditional wet spray method is quite labor intensive and in situations where direct measurement is impossible, thickness control is difficult. In order to provide metallic coatings of substantial thickness, several layers of coating, each approximately two to three mils thick, must be applied to the part to be coated. Prior to applying each subsequent coat, the part must be dried, i.e., the solvent must be evaporated, to prevent dripping and sagging of the coating and to prevent the solvent from becoming entrapped as subsequent layers are applied. Blistering may occur if any solvent remains entrapped when the next layer is applied. After the coating is built up to a sufficient thickness by applying several layers, it is fired to a temperature sufficient to react the cementitious binder or to sinter the metallic binder.
In the case of cementitious binders and especially in the case of sinterable binders, supplemental organic thickeners are needed to prevent settling, to control rheology, and to impart real strength to the coating so as to minimize shrinkage or cracking during drying. The use of these additives may require slow curing of the coated part to remove organic decomposition products without disrupting the coating structure.
Metallic coatings applied using the traditional wet spray method are often applied from aqueous solutions which require increased drying times and impart an increased porosity to the resulting metallic coatings after drying. Moreover, because the metallic coatings are applied as aqueous solutions, it is extremely difficult to make repairs of damaged components. This is especially true when the part requiring repair is large and must be repaired in the field, e.g., an aircraft wing, or when the damaged portion of the coated part is difficult to access.
Using the conventional dry spray method, specialized equipment is required to apply the coatings. Although this equipment is usually available where the initial coating is applied, it may not be available at locations where coating repairs must be made. Also, the extreme heat and velocity used in this method can damage certain fillers incorporated in the metallic formulations by oxidizing or distorting the fillers on the substrate surface as the composition is applied. Such damage can, in certain instances, impair the performance of the coatings.
When using conventional sol-gel technology extreme conditions are required, usually involving very high temperatures and/or pressures. In addition, some material forms and combinations are just not possible using the existing sol-gel techniques because of the limitations imposed by these older techniques.
Moreover, if a metallic coating is to be applied to a smooth surface, additional steps arc required to prepare the surface so that the coating adheres correctly and equally. These steps are required no matter which of the above techniques are utilized. For example, a substrate having a smooth surface must be chemically treated with hazardous chemicals, exposed to extreme conditions or mechanically etched prior to coating. This makes the process even more laborious and therefore more costly.
As stated above, various problems and disadvantages exist with conventional methods for applying metallic coatings to substrates, especially on smooth glass or ceramic surfaces. These problems and disadvantages are overcome by the present invention.
The present invention provides a sol-gel catalyst composition for electroless plating, a process for producing the sol-gel catalyst composition and a method for applying the sol-gel catalyst composition to a substrate to be coated.
The sol-gel catalyst composition for electroless plating of the present invention comprises:
a metal alkoxide in a polar organic solvent;
an acid in an amount of from about 1 percent by weight to about 30 percent by weight of the composition;
a chloride salt or an acid chloride; and
a catalytic metallic salt wherein the metal is selected from the group consisting of Co, Pd, Pt, Ni, Au, and Ag.
The invention also provides a process for producing a sol-gel catalyst composition which comprises:
(a) preparing a metal alkoxide mixture comprising from about 30 percent to about 50 percent by weight of the composition of a metal alkoxide such as tetraethylortlhosilicate (TEOS) and a polar-organic solvent;
(b) combining a portion of the metal alkoxide mixture prepared in step (a) with an acidic solution containing an acid dissolved in a polar-organic solvent and allowing the resulting mixture to hydrolyze and cool to room temperature to produce a stock solution for the sol-gel catalyst composition;
(c) preparing a metalic plating solution comprising about 10 percent to about 20 percent by weight of the plating solution of a chloride salt or an acid chloride, about 30 percent to about 40 percent by weight of the plating solution of a catalytic metallic salt, the salt containing a metal selected from the group consisting of Pd, Co, Ni, Au, and Ag and a concentrated acid; and
(d) combining a portion of the stock solution prepared in step (b) with a portion of the metallic plating solution prepared in step (c) to produce a sol-gel catalyst composition for electroless plating.
The sol-gel catalyst composition produced is relatively stable, having a shelf-life of greater than six months when stored at room temperature in closed containers.
The invention also provides a method for applying a metal coating composition to a substrate comprising:
(a) applying a portion of the sol-gel catalyst composition of the invention to a substrate and heating the substrate to about 100xc2x0 C. to about 500xc2x0 C. for about 10 to about 60 minutes; and
(b) contacting the coated substrate from step (a) with a plating solution for a predetermined amount of time.
The amount of time in which the coated substrate is placed in contact with the plating solution is proportional to the thickness of the plating material deposited on the substrate.
The metal alkoxide used in the sol-gel catalyst composition of the present invention can be any metal alkoxide. Preferably, it is a member selected from the group consisting of tetraethoxyorthosilicate (TEOS), tetramethoxyorthosilicate (TMOS), aluminum isopropoxide, titanium isopropoxidc, zirconium isopropoxide, cerium isoproposide, aluminum butoxide, titanium butoxide, zirconium butoxide, tin 2-ethylhexanoate, tantalum isopropoxide, tantalum ethoxide, indium methoxyethoxide and cerium butoxide. More, preferably, tetramethoxyorthosilicate (TMOS) is used and most preferably tetraethoxyorthosilicate (TEOS) is used.
The metal alkoxide may be mixed or dispersed in any suitable polar organic solvent that has a relatively low boiling point, e.g. isopropyl alcohol. The function of the polar organic solvent in the metal alkoxide solution is to solubilize both the metal alkoxide and the acid. Once the acid is solubilized it causes the sol to gel.
The organic solvent must be volatile, i.e., have a boiling point below about 200xc2x0 C., preferably below 100xc2x0 C., so as to facilitate its removal by a low temperature drying process. Mixing the metal alkoxide in a polar organic solvent that has a low boiling point eliminates the need for slow drying and facilitates the development of unique rheology. Solvents used in the sol-gel include lower monohydric alcohols, ketones, glycols, ethers and mixtures thereof, having a boiling point below 200xc2x0 C. For example, preferred solvents used in the present invention include methanol, ethenol, propanol, butanol, N-methyl-2-pyrrolidinone, 2-methoxy ethanol, 2-methyl-1-propanol, 2-methyl-2-propanol, ethylene glycol, ethylene glycol monoethyl ether, 1-phenol, propyleneglycol monoethyl ether, propyleneglycol monomethyl acetate.
The metal alkoxide mixed with the organic polar solvent is hydrolyzed with acid and stoichiometric amounts of water to form a sol in situ. A wide variety of organic and inorganic acids may be used for this purpose, including nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, phosphoric acid, perchloric acid, formic acid, acetic acid, propionic acid, toluenesulfonic acid, 2-ethylhexanoic acid, benzoic acid, maleic acid, citric acid, lactic acid, oxalic acid, dimethylmalonic acid, malonic acid, chloroacetic acid, dichloroacetic acid, trichloro-acetic acid, trifluoro-acetic acid, trifluoromethane-sulfonlic acid, glutamic acid, glycolic acid and succinic acid.
Once the sol is produced, a metallic solution containing a catalytic metal salt in an organic polar solvent, a chloride salt or an acid chloride, an acid and a stoicheometric amount of water is added to the sol. The addition of an acid to the mixture gels the composition by facilitating hydrolysis of the corresponding metal alkoxide or silica. The catalytic metallic salt that is added to the composition becomes active upon firing and produces a sol-gel catalyst composition. The catalytic metallic salts used in the sol-gel may contain cobalt, nickel, palladium, gold, silver or platinum. Preferably, the catalytic metallic salt used contains palladium or platinum, and most preferably contains palladium.
When using a species containing palladium such as palladium bisacetonitrile dichloride, an aprotic polar solvent is necessary to stabilize the solution. Aprotic solvents that may be used include acetone, methyl-ethyl ketone (MEK), methyl-isobutyl ketone (MIBK), pyridine, dimethylformamide (DMF), dimethylsulfoxide (DMSO), and acetonitrile.
In addition when using palladium and acid that hydrolyzes the sol-gel reaction and stabilizes the palladium by maintaining the palladium species in solution so that any binding with the palladium ions must be released by the firing of the film, may also be used. Of these acids hydrochloric and nitric acids are preferred.
Palladium cations in aqueous solutions tend to hydrolyze and condense much like the metal alkoxides in a sol-gel reaction. Once the particles become sufficiently large then the palladium will precipitate out of the solution. Excess chloride ions in the solution will slow the rate of hydrolysis and increase the stability of the palladium ions. Lowering the pH, preferably by an acid chloride such as hydrochloric acid, will also increase the stability of palladium ions in solution.
Finally, some alkoxides, such as Al (sec-butoxide)3 are so reactive that they require modification by stabilizers such as ethylacetoacetate (etac) to form species such as Al (Osec-Bu)2 (etac) which is a more soluble and less reactive precursor. Titanium isopropoxide is another very reactive alkoxide, which is commonly stabilized using 2-4-pentanedione.
The catalytic metallic salts which are dissolved in the sol-gel may be applied to the substrate to be coated using a number of different ways including spin coating, dipping, painting, brushing, lithography. More, preferably, the sol-gel is applied using spin coating.
After the sol-gel is applied to the substrate, the substrate is fired at a temperature between about 100xc2x0 C. to about 700xc2x0 C., preferably between about 250xc2x0 C. to about 500xc2x0 C. and most preferably between about 300xc2x0 C. to about 450xc2x0 C. to reduce the metal and make the sol-gel catalytic.
Use of the sol-gel catalyst and method for applying a metallic coating of the present invention facilitates electroless plating of thick metal films onto glass and ceramic surfaces without using hazardous chemicals or extreme conditions. The sol-gel catalyst composition can be applied over an entire surface by spinning, spraying or dipping. The catalyst of the present invention may also be applied to selective areas by brushing, printing or silk-screening. Unlike existing methods, no prior etching or conditioning of the surface is required beyond simple cleaning.
The heating process completes the condensation and densification reaction to form an active catalytic film, so that when the catalyst coated substrate is placed in contact with a metallic plating solution for a predetermined amount of time, electroless plating begins immediately. The amount of time that the substrate remains in contact with the plating material is proportional to the thickness applied. In other words, the longer that the substrate is in contact with the plating material the thicker the layer applied.
Once the sol-gel is catalytic it can be applied directly to a substrate having a smooth surface without undertaking some or all of the numerous steps of dipping the substrate into a preconditioning or etching bath; rinsing the preconditioning solution; dipping the substrate into a conventional Pd-Sn bath; rinsing the Pd-Sn solution; dipping the substrate into an accelerator bath which removes the Sn leaving Pd metal on the surface; and finally rinsing the substrate to remove any excess chemicals. The process of the present invention coats substrates having smooth surfaces without having to perform the numerous pre-conditioning steps listed above and therefore makes the process more efficient and cost effective.
Simply stated, the process of the present invention uses very low levels of catalytic metals to coat substrates using the sol-gel of the present invention. This process includes doping the sol gel solutions before it is applied to the surface to be coated. Multiple coatings may be applied to the substrate without the flaking usually associated with such coatings.
The sol-gel catalyst composition produced, bonds strongly to glass, polished alumina or ceramic surfaces and to the catalytic metal as well. It may also be applied to other metallic substrates such as polished copper, tin, steel, or alloys containing several different components. Once the catalyst is applied, the substitute is placed in contact with a plating solution that reacts with the catalyst and plates the substrate. The metal plating applied will then be electrically insulated from the substrate by the catalyst layer. This may be particularly useful to the microcircuit industry. For example, circuit boards and other electrical components that need to be insulated from the substrate may be produced by the method described above.
Other substrates that may be coated with the composition of the invention may include airplane parts, such as, wings, hatches, fins, tails, and other smooth surfaces associated with the aircraft industry.